The present invention is directed to a process for the production of phosphoric acid by the wet process. Phosphoric acid has been prepared by the wet process for many years. The wet process involves the reaction of phosphatic solid materials, hereinafter termed phosphate rock, with sulfuric acid, usually in a slurry of phosphate rock and calcium sulfate in phosphoric acid. From an overall view, the sulfuric acid reacts exothermicly with the phosphate rock to produce phosphoric acid and calcium sulfate; however, an intermediate exothermic reaction is that of phosphate rock with phosphoric acid to produce monocalcium phosphates. Furthermore, the preparation of calcium sulfate hemihydrate or dihydrate is slightly endothermic.
The names of the three processes are based upon the by-product calcium sulfate produced; namely, the gypsum or dihydrate process, the hemihydrate process and the anhydrite process. The type of by-product produced is dependent upon the temperature of the system and the P.sub.2 O.sub.5 concentration of the liquid. Other factors such as fluorine concentration, alumina concentration and sulfate in concentration play a less important role.
As used herein, "calcium sulfate" refers to all three types of calcium sulfate, i.e. gypsum, hemihydrate, and anhydrite.
Gypsum, CaSO.sub.4.2H.sub.2 O, is the by-product formed when the wet process is run at a temperature of 90.degree. C. or less and a P.sub.2 O.sub.5 concentration of about 30% in the liquid portion of the slurry. Increasing the temperature to about 90.degree.-120.degree. C. and the P.sub.2 O.sub.5 concentration to about 40% in the liquids phase yields hemihydrate, CaSO.sub.4.1/2H.sub.2 O. Adjusting the temperatures and concentrations, for instance, to 75.degree. C. and 40% P.sub.2 O.sub.5 results in a mixture of gypsum and hemihydrate which is very unstable. An unstable system such as this causes trouble during filtration due to the hardening or setting-up of the gypsum-hemihydrate solid on the filter. Care must be exercised in maintaining the proper temperature and P.sub.2 O.sub.5 concentration in the process being run in order to avoid such problems. CaSO.sub.4, anhydrite, is produced at temperatures of about 130.degree. C. at P.sub.2 O.sub.5 concentrations greater than 30%. This latter process is most difficult to run due to the severe corrosion at the higher temperatures and the instability of the anhydrite during processing.
Because the overall reaction between sulfuric acid and phosphate rock is exothermic, provisions can be made to remove heat from the system at a preselected temperature of the reaction system. This has been accomplished by (1) blowing air through the slurry or (2) pumping a portion of the slurry to a vessel under vacuum or (3) conducting the operation in a vessel under vacuum. The first method, use of air as a coolant, is undesireable because it is necessary to scrub large amounts of air exiting the system to remove pollutants, mainly fluorine in the form of hydrogen fluoride or silicon tetrafluoride. The equipment required for scrubbing is quite expensive.
In the second method, described in U.S. Pat. No. 2,699,985, a portion of the hot slurry is removed from the main body of the slurry, and subjected to vacuum. Cooling occurs by evaporation of water. The cooled slurry is recycled to the main body of the hot slurry and moderates the temperature of the process.
A third method, conducting the reaction under vacuum, has many desirable features. For example, the cooled slurry is immediately dispersed within the hot slurry and temperature differentials within the slurry are minimized. In addition, the slurry is concentrated by the removal of water, and the desired temperature is easily maintained.
Problems associated with the calcium sulfate dihydrate (gypsum) process include the incomplete reaction of phosphate rock, poor filterability and washability of the by-product calcium sulfate, and coprecipitation of calcium phosphate with the calcium sulfate. Such problems can occur when employing the hemihydrate process.
For example, incomplete reaction of phosphate rock can result from precipitation of calcium sulfate dihydrate on the surface of the phosphate rock. This deposit impedes digestion of the rock with sulfuric acid or phosphoric acid, resulting in the undissolved rock which is passed to waste disposal. This deposit is caused by an excessively high local concentration of sulfate ions in the presence of the phosphate rock.
Filtration of the slurry produced in the gypsum process can be slow. If and when, due to reaction of phosphate rock with phosphoric acid, increased cooncentrations of calcium phosphates and sulfuric acid occur in proximity to each other, many small crystals form as product solids. A situation arises in which a high number of small particles are formed in the system. Increasing the residence time does little to improve particle size. The rate of filtration of the slurry containing these small crystals (or "fines") is drastically reduced.
Coprecipitation of dicalcium phosphate (CaHPO.sub.4) with calcium sulfate dihydrate can occur in the presence of localized high concentrations of monocalcium phosphate Ca(H.sub.2 PO.sub.4).sub.2. This results in loss of phosphate values because the calcium becomes part of the calcium sulfate dihydrate crystal structure. As such, it cannot be washed out of the crystal structure during subsequent separation and washing operations and it passes to waste disposal.
Attempts to alleviate the problem of poor dispersion or localized high concentrations of reactants are many. In use today are the one slurry system and the multi-slurry system for the production of phosphoric acid by the wet process. Circulation within each vessel and circulation between vessels is desirable.
In one slurry process, the phosphate rock and the sulfuric acid are added to the slurry in one tank. Agitators, in union with baffles are used to circulate the slurry into which the reactants (phosphate rock and sulfuric acid) are added. To the extent that the localized concentration differences are minimized the slurry has only one sulfate level. This is undesirable because improved yields are obtained when the phosphate rock is dissolved at a lower sulfate concentration than at which calcium sulfate crystallizes.
A multi-slurry system can be of two types. Two or more compartments or cells can be constructed within one large vessel, the compartments being interconnected in series, or multi-vessels can be used. For the multi-compartment scheme, the reactants are added separately, that is, in different compartments in order to increase the dispersion of the reactants in the slurry. At the last compartment, some slurry is removed from the system for recovery of phosphoric acid. The major portion of the slurry is recycled from the last compartment to the first compartment.
A multi-vessel process involves the use of two or more connected vessels. The reactants are added to the slurry in separate vessels so as to more completely disperse one reactant in the slurry before it is contacted by later added reactants. Often the system is arranged so that slurry is recycled from the last vessel back to the first.
There is a need for a method and apparatus for producing phosphoric acid by the wet process where the problems of incomplete reaction of phosphate rock, poor filterability and washability of the by-product calcium sulfate, and coprecipitation of calcium phosphate with by-product calcium sulfate, are avoided.